A liquid crystal display device including a reflective region in which pixels each provide display in a reflection mode and a transmissive region in which pixels each provide display in a reflection mode is referred to as a transmission-reflection combination type or transreflective type liquid crystal display device. A transreflective type liquid crystal display device includes a backlight, and can provide transmission mode display using light from the backlight and reflection mode display using ambient light at the same time or provide either type of display in a switched manner. Such a transreflective type liquid crystal display device is widely used especially as a small or medium size display device for mobile apparatuses used outdoors.
The transreflective type liquid crystal display device conventionally adopts a structure in which a liquid crystal layer has a smaller thickness in the reflective region than in the transmissive region (occasionally referred to as the “multi-gap structure”) in order to improve the display quality in the reflection mode and the transmission mode. It is most preferable that the thickness of the liquid crystal layer in the reflective region is ½ of the thickness of the liquid crystal layer in the transmissive region. Light which contributes to the display in the reflection mode passes the liquid crystal layer twice. Therefore, by setting the thickness of the liquid crystal layer in the reflective region to ½ of the thickness of the liquid crystal layer in the transmissive region, the retardation caused by the liquid crystal layer to the light used for the display in the reflection mode matches the retardation caused by the liquid crystal layer to the light used for the display in the transmission mode. As a result, a voltage-luminance characteristic which is optimum for both of the reflective region and the transmissive region is obtained.
In a transreflective type liquid crystal display device of a multi-gap structure, a step is formed in the pixel in order to reduce the thickness of the liquid crystal layer in the reflective region. For example, with the structure described in Patent Document 1, an interlayer insulating layer is provided below a reflective electrode in a TFT substrate, and thus the thickness of the liquid crystal layer in the reflective region is made smaller than the thickness of the liquid crystal layer in the transmissive region by the thickness of the interlayer insulating layer. By contrast, a structure in which the thickness of the liquid crystal layer in the reflective region is reduced by providing a transparent resin layer in the reflective region of a color filter substrate, which is located on the viewer side of the liquid crystal layer so as to face TFTs, is also known (e.g., Patent Document 2).
Meanwhile, as a technology for controlling pretilt directions of liquid crystal molecules, Polymer Sustained Alignment Technology (hereinafter, referred to as the “PSA technology”) has been recently developed (see Patent Documents 3 and 4, and Non-patent Document 1). The PSA technology is as follows. A small amount of polymerizable material (e.g., a photopolymerizable monomer) is incorporated into the liquid crystal material. After a liquid crystal cell is assembled, the polymerizable material is irradiated with an active energy beam (e.g., ultraviolet rays) in the state where a prescribed level of voltage is applied to the liquid crystal layer. By the generated polymerization product, the pretilt directions of the liquid crystal molecules are controlled. The alignment state of the liquid crystal molecules at the time when the polymerization product is generated is sustained (stored) even after the voltage is removed (in the absence of the applied voltage). Accordingly, the PSA technology has an advantage of capable of adjusting the pretilt azimuths and the pretilt angles of the liquid crystal molecules by controlling the electric field or the like formed in the liquid crystal layer. In addition, the PSA technology does not require rubbing and so is especially suitable for a liquid crystal layer of a vertical alignment type, in which the pretilt directions cannot be easily controlled by rubbing. Patent Documents 1 through 4 and Non-patent Document 1 are entirely incorporated herein by reference.